Sealants are widely used in building and civil engineering works as they serve several general functions as follows:
(a) Waterproofing Function
Sealants are used as components of complete waterproofing system by resisting the passage of water into joined part of the building.
(b) Environmental Function
Sealants are used as barriers to resist the passage of heat, light, sound, odor, dust, etc. into the building.
(c) Movement Control Function
Sealants are used to accommodate continuing changes in size of the joint due to thermal, moisture and structural movements, including vibration and creep.
A sealant must be able to perform these functions. At the same time, it must have acceptable appearance, durability, economic and maintenance cost. In some cases, additional functions may be required such as ability to perform under special temperature or humidity conditions and/or to withstand a polluted atmosphere.
Polyurethane as a high performance sealant is widely used in joints of curtain walling on building facades. Although highly polymeric, it is susceptible to degradation due to environmental factors such as heat, moisture, UV and others. These factors either alone or in combination affect the performance of the material in the process of curing as well as during long term exposures. In the tropics, the material also tends to fail prematurely if exposed excessively to heat and water. High temperature for example may change the physical structure of polyurethane by loosening inter-macromolecular interactions and change the chemical structure of polyurethane through the break-up of macromolecular chains and the formation of low molecular mass products. The polyurethane sealant degradation at high temperature in the presence of moisture or water may proceed through water-swelling, loss of inter-macromolecular interactions, hydrolysis of urethane linkage and chain degradation.